The invention generally relates to preformed articles formed of a brazing foil, as well as methods for the manufacture of assemblies which include said preformed articles.
Brazing is a process for joining parts, often of dissimilar compositions, to each other. Typically, a filler metal that has a melting point lower than that of the parts to be joined together is interposed between the parts to form an assembly. The juncture of the assembled parts and the filler metal is then heated to a temperature sufficient to melt the filler metal but generally lower than the melting point of the parts. Upon cooling, ideally a strong, void free joint is formed.
Brazing in widely used in the manufacture of various assemblies, which themselves may be finished articles which may be used or sold, or which assemblies may be components of such articles. One class of products produced by brazing processes are heat exchangers. Such may take a wide variety of configurations, but those which are referred to as shell-and-tube type, and plate type heat exchangers are most usually encountered. In the former configuration, a larger diameter housing typically referred to as a xe2x80x9cshellxe2x80x9d and encompasses one or more small diameter xe2x80x9ctubesxe2x80x9d or pipes. According to this configuration, a first fluid (liquid, gas) passes through the shell and about the exterior of the tubes while simultaneously a second fluid (liquid, gas) passes through the interior of the tubes. While no physical contact is permitted between the first and second fluids, heat transfer occurs across the tubes. In plate heat exchangers, one or more plates separate a first fluid from a second fluid while heat transfer occurs across the plate. In these types of heat exchangers (as well as in other assemblies), metals are most commonly used due to their high strength and good heat transfer characteristics. Typically, the individual parts which are used to make up these types of heat exchangers are joined by brazing. Thus, it is imperative that joints exhibit high strength, and be resistant to potential detrimental effects which might result from contact with one or both of the fluids.
In order to meet these requirements, the materials of construction for heat exchangers may be carefully selected. Stainless steels are very commonly encountered in heat exchangers as they exhibit advantageous properties including good mechanical strength and good corrosion resistance. Nonetheless other metals also find use in heat exchangers as well.
The manufacture of heat exchangers, especially those of the shell-and-tube variety is frequently very labor intensive, often requiring a significant number of manual assembly steps. For example, in shell and tube heat exchangers a plurality of tubes are inserted through a plate which has suitably sized holes for accepting the tubes. In order to form a pressure tight seal between the tubes and the plate, the junction between these elements need to be brazed. Typically, a brazing xe2x80x9cpastexe2x80x9d composition is used. Such a conventional brazing paste composition includes a brazing filler metal in a powder based form in conjunction with an organic binder as a carrier. In use, this brazing paste composition is deposited in the region of the junction between each tube and the plate. Subsequently, once the application of the aforesaid brazing paste composition is complete, the assembly is then brazed under appropriate conditions in order to drive off the organic binder and simultaneously to form a brazed joint between each individual tube and the plate used in the shell and tube heat exchanger. This operation is however unreliable as it is difficult to deposit equal amounts of brazing paste composition in each individual joint area. It also results in substantial joint porosity due to poor fusion of the powder particles. In addition this manually controlled application process is frequently time-consuming, requires a great deal of manual skill, and requires very controlled handling during the complete process. Many risks are associated with such an assembly process, each of which will result in a failed joint in the brazedassembly.
One potential technical risk lies in the very nature of the brazing filler metal composition which is used. As this must be spread at the joint, and retained there, it is inevitable that a thickening agent, typically one or more organic materials need be present. These ideally are driven off during the heating stage when brazing actually occurs, but it is well known in the art that very frequently bubbles, fractures, or other discontinuities in the brazed joint result. Such of course are defects in the completed, and brazed assembly. In order to repair such defects, a rebrazing step is necessarily practiced in order to repair specific defects. Another technical risk attendant upon the current method of production lies in the fact that a very uniform distribution of the brazing material should be placed at the junction of each tube and the plate. This is necessitated as ideally, the thickness of the brazes should be essentially uniform so that under pressurized operating conditions, weak joints do not fail during the useful service life of the heat exchanger. Naturally, this may be difficult to perform reliably in a manual application process such as is currently known in the art. Occasional excessive paste deposition may result in excessive erosion or dissolution of the thin walls of the tubes, and may even lead to formation of holes in the tube walls. A still further technical risk which exists relates to the handling of the shell and tube assembly subsequent to or during the application of the brazing filler metal as well as the composition of the brazing filler metal. Typically the brazing filler metal used in such a manual operation includes a minimal amount of an organic binding agent and/or other organic materials. Due to the known problems associated with the presence of such organic materials, a minimization of their presence is desirable. Unfortunately, minimization of such undesirable organic materials also deleteriously affects the spreadability well as the adhesive characteristics of such a brazing filler metal comprising composition. Thus, it is not unknown that dislocation or movement of the one or more tubes on the plate during the assembly process at any point up until the formation of the brazed joint can occur. This is particularly likely where large number tubes are to be assembled and especially where two plates in opposite ends of the tubes need be concurrently assembled. Again, any failure in the placement and retention of the brazing filler metal composition up until the actual formation of the brazed joint also manifests itself in failed brazed joints in such heat exchangers. A still further technical risk which is present is the difficulty in forming small heat exchangers wherein one or more relatively small diameter thin walled tubes need to be brazed to one or more end plates. Fabrication of assemblies of small diameter, thin walled tubes are troublesome as they require a very precise application of a brazing filler metal composition at the juncture between each tube and each plate. At the same time, in such small heat exchangers, it is also very desirable to use only a very minimal, but sufficient amount of the brazing filler metal to form each joint. Unfortunately, it is known to be difficult to place precise amounts, in precise locations, of brazing filler metals even with the use of syringes, or other type of delivery apparatuses. From the foregoing it is apparent that there is a real and continuing need in the art relating to the production of brazed assemblies, particularly shell-and-tube heat exchangers.
Accordingly, it is to one or more of these technical needs that the present invention is directed.